Three-piece chair backrest frame



FIG. 1 is a perspective view of a three-piece chair backrest frame,showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof.

The ornamental design for a three-piece chair backrest frame, as shown and described. 